In recent years, injectors for fuel injection for automobiles using laminate-type piezoelectric elements have been developed from the standpoint of improving fuel efficiency of automobiles and from the environmental point of view, such as reduced exhaust gas emission.
The laminate-type piezoelectric element has a ceramic laminated body obtained by alternately laminating piezoelectric layers usually made of a piezoelectric material and inner electrode layers having electrical conductivity, and has outer electrodes joined to the side surfaces of the ceramic laminated body via an electrically conductive adhesive. When a voltage is applied across the inner electrode layers, the piezoelectric layers undergo a displacement so as to be driven.
When the laminate-type piezoelectric element is used, for example, for an injector, a variety of problems arise in severe environments, such as being used for extended periods of time in a high-temperature atmosphere. For example, when the element is driven, i.e., when the ceramic laminated body is contracted and elongated in a direction of lamination due to piezoelectric displacement, stress is exerted on the electrically conductive adhesive arranged on the side surface thereof. When the stress is repetitively exerted, a problem arises in that cracks develop in the electrically conductive adhesive. Therefore, electric conduction (hereinafter simply referred to as conduction) is not maintained between the inner electrode layers and the outer electrodes which are electrically conductive to each other via the electrically conductive adhesive, and problems such as poor conduction may arise.
In order to solve the above problem, for example, Japanese Unexamined Patent Publication No. 2003-086853 discloses the structure of a laminated-type piezoelectric, element in which metallic plate-like conducting members are joined to the side surfaces of the pole-like laminated body (ceramic laminated body), and an electrically conductive adhesive is filled between them. Further, Japanese Unexamined Patent Publication No. 2001-244514 discloses the structure of a laminate-type piezoelectric actuator in which a thin metal plate or a metal mesh is buried in the electrically conductive adhesive provided on the side surfaces of the actuator body (ceramic laminated body). According to this document, stress generated in the electrically conductive adhesive can be relaxed while the piezoelectric element is being driven.
As the electrically conductive adhesive, however, there is usually used one containing an electrically conductive filler in a resin material which is a chief component. Therefore, characteristics (e.g., specific gravity, thermal expansion, etc.) differ greatly between an electrically conductive adhesive using resin material as a chief component and the thin metal plate, the metal mesh or the above plate-like electrically conducting member which comprises a metal material. Therefore, when the piezoelectric element is driven, stress occurs between any of these two elements which are in contact, and cracks occur in the electrically conductive adhesive.
That is, in the conventional structure, the effect of relaxing the stress that occurs in the electrically conductive adhesive when the piezoelectric element was being driven, and of suppressing the occurrence of cracks was not sufficient.